Even more Combinatorics Problems (Based on set {0,1,2,3,4,5,6}

28. a) **How many 4-digit numbers can be formed from the set A = {0,1,2,3,4,5,6} if there is no repititon?**

***Don't need help solving this... But it has to do with the other questions I need help with ***

**c) How many of the numbers in part a) contain a 3?**

* I know that numbers such as 0321 can't count because that actually represents the number 321. This is my thinking process thus far.

First digit (6 choices - excluding 0)

2nd digit (6 choices - can't be the 1st digit choice)

3rd digit (5 choices - can't be the 1st of the 2nd digit choice)

4th digit - I know I have to incorporate where the 3 is? I was thinking about adding it to the total?

If someone could show me how to solve this *correctly* that would be AWESOME!

**Also need help with this... d) How many of the numbers in part a) are divisible by 5?**

**- **The examples I have in my textbook refer to dividing many numbers at a time, (Using the Exclusion/Inclusion Principle) - which is why I am confused on what to do when there is just one number to divide.